Through the efforts of the ATM Forum, a protocol standard that enables the building of multivendor, interoperable ATM switching networks has been developed. The protocol standard relates to ATM connections within the Private Network-Network Interface (PNNI) domain. The protocol allows for the building and maintenance of large ATM networks because it provides hierarchical, dynamic link-state routing instructions and communication pathways. The PNNI routing is based on well-known, link-state routing techniques. The PNNI provides support for quality of service (QoS) routing required for applications with real-time requirements and scalability to large global networks. These features provide large-scale networks with a single routing protocol. In these environments, rerouting refers to changing the call path of an existing call connection anywhere between its two end-points (i.e., the source and the destination). No user signaling is needed to reestablish the connection as the operation is performed by the network. This permits the calling and called parties to be continuously connected. In accordance with known signaling procedures, in the event of a network failure, the connection is rerouted within the PNNI domain, i.e., between the two user-to-network interfaces (UNI) connecting the calling and called party.
The PNNI network may be organized into one or more rerouting domains, with each node potentially belonging to at most one rerouting domain. The Domain-based Rerouting (DBR) standard defines connection recovery and path optimization procedures within the context and scope of each rerouting domain (termed the local rerouting services). The connection recovery provides protection from a link failure (“fault protection”) and connection segment release containment and recovery within the rerouting domain, thereby ensuring that the overall connection remains in the connected state. Only minimal cell loss on the data or call path is incurred as a result of connection recovery. Also, the path optimization ensures maximum use of network resources and bandwidth by allowing an existing connection to optimize onto a better or desirable path without the overall connection being released.
However, in the networks supported by the DBR standards, when a connection recovery fails within a rerouting domain locally, the connection will be released. Therefore, current best behavior by the nodes is to release the connection and attempt connection setup again. However, the end user's are impacted by being disconnected. Likewise, when a network resource that is not part of a rerouting domain fails, the connection is released. It is possible to ensure all network resources are protected by a rerouting domain, however in this case such resources are only singly protected. Therefore, it is preferable that all resources are doubly protected. Also, according to the DBR procedure, when path optimization is attempted, resources outside of the rerouting domain are ignored. It is possible to ensure the entire network is a single rerouting domain. This will, however, impact connection recovery in large networks where the connection recovery service is made scalable by dividing the network into separate rerouting domains.